The balloon Melange, northwest Nelson: Origin, structure, and emplacement

Abstract

New work on the Balloon Melange, a regional scale body of disrupted sandstone and mudstone that encloses a number of exotic blocks, suggests that the characteristic fabric has a two‐stage origin. The first stage is dominated by layer parallel extension in weakly lithified rocks, and the second by further strain and cleavage development in lithified rock. Location of intrusive contacts between melange and arc assemblage rocks suggests that initial disruption was followed by a diapiric intrusion into a Cambrian volcanic arc plus arc‐derived sediment host, probably in the latest Cambrian. Mesoscopic and microscopic structural examination shows the fabric was extensively modified during the development of S ₁ slaty cleavage. Cleavage formation postdates diagenesis in both melange and host and probably developed in the mid Paleozoic.

Petrology, geochronology, and geochemistry confirm that quartzofeldpathic sediments similar to the Junction Formation were the dominant contributors to the melange. The quartzofeldspathic suite (Junction Formation and Balloon Melange protolith) has an Australian‐Antarctic provenance and is thought to be part of a submarine fan that may have been incorporated into an accretionary complex developed near the trench at a convergent plate boundary. Further crustal shortening is required to place the volcanic arc assemblage above the accretionary complex prior to diapiric injection of the melange. Subcretion of water‐rich sediments above a subduction zone would create an environment predisposed to diapiric intrusion. Scale, and the juxtaposition of rocks of backarc, arc, and accretionary complex origin, points to significant shortening after arc volcanism ceased.

The architecture of the Southwest Pacific sector of the Cambrian Gondwana margin and plate boundary is not well understood. Crustal shortening and thrusting are widely developed in the Late Cambrian to Early Ordovician Ross‐Delamerian orogen, and the events leading to the emplacement of the Balloon Melange suggest that the Ross‐Delamerian orogeny affected northwest Nelson.

Keywords:

• Cambrian
• Nelson
• Balloon Melange
• structure
• tectonics
• cleavage
• zircon ages
• Takaka Terrane